Pigment manufacture



Patented Jan. 19, 1937 UNITED STATES- PATENT. OFFICE I 2,068,039rrcMEN'r MANUFACTURE Harold Robert Rafton, Andover, to Raffold Process.Corporation,

oi! Massachusetts Mass, assignor a. corporation N Drawing- ApplicationJanuar 26, 1932, Serial N0. 589,045 I 12 Claims. (oi. 13 1- -58) Myinvention relates 'to a pigment and-the manufacture thereof.

The principal object of myinvention is the manufacture of pigment havingthe composition 5 ofcalcium carbonate magnesium basic carbonate.

A further objectof my invention is the. manucarbonate magnesium basiccarbonate substantially free from long, needle-like, or featherycrystals. A further object is the manufacture of calcium carbonatemagnesium basic carbonate by a simple and economical process.

Other objects and advantages of my invention will become apparent during.the course of the 2 following description.

In my Patent No. 1,595,416 issued August 10,"'

1926, I- disclose a paper filled with calcium carbonate magnesium basiccarbonate and also a paper coated with calcium carbonate magnesium basiccarbonate.-

I describe in that patent a method of producing calcium carbonatemagnesium basic carbonate by treating a lime containing magnesia'withcarbon dioxide (page 2, lines 29-101 inclusive). In the above describedmethod the calcium car'- bonate magnesium basic carbonate may beproduced from a lime containing magnesia preferably by slaking the limethoroughly in water and then subjecting it to carbon dioxide in excessof the amount required to satisfy the calcium. One modification of thematerial (which I- 'may call modification A)- is produced by allowing'sufllcient carbon dioxide to react up to the point where, for examplewith a dolomitic lime, i. e. a lime containing approximatelyequimolecular proportions of calcium and magnesium, the calcium wouldjust begin to dissolve as bicarbonate. It

. may be that the dolomitic lime contains practically'the highestpercentage of magnesia of any lime containing magnesia commerciallyavailable. When such a reaction is carried out under pressure at asuitable concentration, about two-thirds of the magnesium can'bedissolved before the calcium begins to go into solution. A convenientmethod of carrying out this reaction is stated to be in the cold' underpressure. The undissolved materialhas the desiredcomposition of; calciumcarbonate magnesiumbasic carbonate, and is the above referred tomodification A. This modification, thus, contains less magnesium inproportion to the calcium than there was in the original lime used asstarting material.

If desired, the magnesium containing liquid may be treated for therecovery of the dissolved magnesium, and the recovered magnesiumcompound subsequently mixed with the above men-' tioned undissolvedmaterial, giving what I may term modification B1; or the recovery of thedissolved magnesium may take place in the presence of the undissolvedmatter, giving modification B2. In either case the material producedcontains a mixture of the original undissolved material and themagnesium material precipitated from its solution, the compositematerial being very similar in the two cases, and thus they may both begrouped as modification B.

A further alternative procedure is to continue the addition of carbondioxide after the calcium begins to be dissolved and subsequentlyrecover the entire calcium and magnesium from solution, or in additionto any which may remain undissolved, by removal of the carbon dioxidewhich is holding them in solution. The material so obtained I may termmodification C. It will be notedthat modifications B and C containsubstantially the same proportion of calcium to magdid the lime fromwhich they were nesium as made.

I have found that if modification A be made from lime in moderatelydilute aqueous suspension, e. g. approximately 5 .per cent., moreorless, a material may be produced which is finely divided, andsubstantially all of whose particles appear to be of fine point-likeshape under the microscope. I may term this modification A1.

Under other conditions, such particularly as when the aqueous limesuspension is considerably more concentrated, I have found that theremay also be seen under the microscope, in addition to the point-likeparticles, a certain amount of crystalline material either long orneedle-like, or.

of somewhat feathery shape, which is probably a compound of magnesiumprecipitated from solution. Thesecrystals' are, of course, very finelydivided from the standpoint of; examination by macroscopic means andalso when viewed under the microscope, but when viewed under the higherpowers they are readily distinguished from the point-like particles.tion A2.

Modifications B and C, when viewedunder the microscope, are somewhatlike modification A2, with the exception however that modifications Band C have a great deal more of the needle shape or feathery crystalspresent.

It will be noted that of the various calcium carbonate magnesium basiccarbonates made by the several procedures above, only modification A1possesses the point-like microscopic structure, and this structure isthus obtained only when the calcium carbonate magnesium basic carbonatecontains less magnesium in proportion to the calcium than is containedin the original lime. As, in the illustrative, example of the manufacture of modification A1 given above, approxi- I may term thismodificamately two-thirds of the original lime used has been dissolvedout, the calcium and magnesium radicles arethus present in modificationA1 in the molecular proportions of substantially three to one.

I have 'found that by conducting the manufacture of calcium carbonatemagnesium basic carbonate in a somewhat different manner from either ofthe illustrative procedures referred to, I can produce a calciumcarbonate magnesium basic carbonate, which, although it containssubstantially the same proportion of magnesium to calcium as obtainsinthe-lime containing magnesiaoriginally employed, nevertheless containssubstantially no needle or feathery shaped crysbonates or carbonates andhydroxides, as the particles of the former material althoughmicroscopically minute, either do not have the pointlike structure of mynew modification, or possess needle-like crystals, or both.

Moreover, my new modification possesses other characteristics distinctfrom the calcium carbonate magnesium basic carbonate produced by eitherof the illustrative procedures referred to. It also produces filled andcoated papers of characteristics distinct from the papers made from thecalcium carbonate magnesium basic carbonate produced by the proceduresabove referred to. Moreover, the method of manufacture of such materialis in some respects considerably simpler than the manufacture ofmaterial by any of the above quoted adaptations of the former method,and the equipment required for the production of the material is in somerespects considerably simplified.

7 reaction in the cold and under pressure. In'

In my former patent, in that procedure where in lime was subjectedto theaction of 'carbon dioxide, I stressed the desirability of performing thereaction under conditions favoring the absorption of carbon dioxide,that is, under conditions favoring the formation of magnesiumbicarbonate. Thus, Irecommend carrying out the my new method, on thecontrary, I carry out the reaction under conditions which are unfavorofthe atmosphere is more conveniently and economically employed. Underthese conditions of high temperature, and pressure substantiallyatmospheric, I find that substantially no magnesium bicarbonate solutionis formed. The calcium carbonate magnesium basic carbonate ob tained asthe reaction product contains no, or substantially no, long needleshaped or feathery crystals, but appears to be a magma of homogeneousfinely divided particles of point-like shape.

The microscopic appearance of this material is not essentially differentfrom the microscopic appearance of modification A1 but nevertheless thematerial has characteristics of increased opacity and the capability oftaking a higher finish distinct from this former material, and impartsdistinct technical differences to the papers manufactured therewith.This may be due to the somewhat difierent basic compound which maycomprise the basic magnesium carbonate which is formed by my new methodand probably also to the proportionately greater magnesium compoundcontent of my new material.

The new material difi'ers markedly, however,

in microscopic appearance from modification A2 and particularly frommodifications B and C, as well as from the modifications produced fromthe. precipitation of soluble salts (as indicated above), and givesdistinct technical differences in the papers manufactured therewith,notably a higher opacity and an improved finish.

I have stated above that in,the manufacture of my material, I prefer tocarry out the reaction at substantially atmospheric pressure.Temperature, however, is the important condition to be observed inmaking my new material, the matter of pressure playing a secondary role.My material can in fact be produced at pressures greatly in excess ofatmospheric, but under such circumstances, in order substantially toavoid the formation of crystals other than point-like in shape, thenecessity for careful temperature control is greater than atatmospheric-pressure. It is necessary under such conditions to minimizeas much as possible the solubility of the carbon dioxide in the water inwhich the lime suspension is held, and this is best done by keeping thetemperature as near to boiling as possible. On the other hand, wheresubstantially atmospheric pressure is used, or even subatmosphericpressure,

a reasonable latitude in the temperature can be observed.- As a matterof fact, I have found that at substantially atmospheric pressure thetemperature of the reaction may be as low. as approximately C.- (and atsubatmospheric pressure somewhat lower) without the production ofsubstantially any needle or feathery shaped'crystals in my material.However there is a small.

' advantage of handling material suspended in a magnesium containingliquid, particularly if it is desired to subject the material to vacuumfiltration, in which case the deposition of the inaterial from solutioncauses considerable trouble by blinding the filter cloths. Thus,although as r 1 stated above a somewhat inferior grade of my -'materlalmay be made at somewhat lower temperaturea'I prefer to carry out thereaction at a temperatme as near boiling as feasible.-

aoeaoso- If however the reaction be carried out so that a magnesiumcontaining liquid such as magnesium.

or preferably by .the addition of a material forming with the dissolvedcarbon dioxide, or the carbonate or bicarbonate ion, an insolublematerial. Examples of suitable addition materials are'alkaline compoundsof alkaline earth metals, such as lime, preferably slaked limecontaining magnesia, calcium carbonate magnesium hydroxide, or a mixtureof the same with lime.

In'the preferred method of making my new material I start with a limecontaining magnesia. I do not restrict myself to lime of any definitemagnesia contentas I have found limes of various magnesia contentssuitable for my purpose. Inasmuch, however, as dolomitic lime, i. e.lime which contains calcium and magnesium in approximately equimolecularproportions, is a common and inexpensive form of lime containingmagnesia, and inasmuch as a high percentage of magnesia seems to enhancetheeuality of my material, that type of lime is a preferred type to usein my process.

I preferably thoroughly slake the dolomitic lime with water, preferablyhot water, and particularly hot .water which is boiling or nearlyboiling. The most eflicient slaking is obtained when the slaking iscarried out thick, and I thus prefer to use a quantity of water inslairing which will result in a final slaked mass of the character of athick mud. This quantity of water will vary with different limes used,but may readily be determined by experiment. I prefer to agitate themass during slaking and preferably to continue the agitation for sometime after the slaking action has apparently ceased in order that theslaking may really be brought to a completion; Ordinarily two to threehours is a sufficient time in which to complete the slaking action.Although any suitable apparatus may be used for the slaking, I prefer touse a tank provided with an efiicient agitating device. v

I next preferably dilute the slaked lime with water to a consistencywhich may vary with conditions, but which may conveniently be in theneighborhood of 1 to 2 lbs. dry solids to the galion. The presence of aminimum quantity of water is desirable as this is a condition which isunfavorable to the formation of magnesium bicarbonate in the reactioHowever from a practical operating standpoint the quantity of waterpresent should not be reduced too far, and should be preferably suchthat, after the reaction has been completed, the final reaction mix isstill sufficiently liquid to flow readily and be capable of beingpumped. The concentration of approximately 1 to 2 lbs. per gallonrecommended above is such as to give, with most limes, a final reactionmix of a desirable low water content, but still of such a fluidity as tobe capable of being p mped.

The suspension of slaked lime at approximately l to 2 lbs. per gallon isthen preferably raised to a temperature above 60 C., preferably notbelow approximately C., and better as near C. as feasible, and at suchtemperature is sub- .centration, agitation,

- structions are well vlde means agitators, a particularly jected tocarbon dioxide gas, preferably until no furthe'r'carbon dioxide isabsorbed. For purposes of this specification substantially 100 C. istaken as the boiling point, although this as will be understood willvary according to atmospheric conditions, altitude, pressure whetherpurposely reduced or increased as compared with atmospheric, thepresence or absence of any dissolved salts in the slurry, and the like.The temperature range which may be used however will normally be withinapproximately 20 C. of the boiling point. The rate of absorption of thegas is high at first as no doubt the calcium reacts with the carbondioxide until it is substantially all completely satisfied, i. e.converted to calcium carbonate, before the magnesium is substantiallyacted upon. The magnesium then begins to be acted upon however and thereaction proceeds to completion, i. e. until substantially all themagnesium content of the lime is converted to magnesium basic carbonateofa definite composition as explained below, after which substantiallyno further carbon dioxide is absorbed. As will be apparent to oneskilled in the art the time required will vary greatly with conditionsof conexcess of gas and the like, but by adjusting these variableswithin economic limits I have been able to conduct the reaction in from8 to 24 hours. I do not however limit myself to such times as they maybe varied as desired according to conditio Inasmuch as the reactiontakes an appreciable time, for the purpose of conserving gas, I preferto carry out the reaction manner, that is, byexposing the fresh carbondioxide gas to the almost completely carbonated lime suspension,allowing the almost exhausted gas to come in contact with thepractically uncarbonated lime. Although any suitable apparatus may beused for the gas absorption, a very convenient equipment for carryingout this reaction, providing it is desired to make the process acontinuous one, is a tower provided with a pluralityof horizontalcompartments, into the top compartment andsuccessively to the bottom oneof which the solids in suspension flow, finally leaving from the bottomcompartment of the tower; and into the bottom compartment of whichcarbon dioxide is introduced, and into the successively highercompartments of which it passes, until any residual gas remaining passesout of the top of the tower. The suspension is maintained in eachcompartment preferably at a substantial depth, conveniently one to threefeet, and the gas is passed successively from the top of a lowercompartment into the suspension in the next. Towers of this type ofseveral con- In order to assist the reaction, I prefer to prointimatelyto mix the gaswith the suspension of solids in the several compartments.Suitable means for such purpose are high speed suitable type being thatcomprising a horizontal revolving turbine wheei attached to a verticalshaft, cooperating with a fixed peripherally arranged series of vanes.Several of these agitating devices are on the market, and I have foundthe type known commercially as the fTurbo-mixer to -be satisfactory.Preferably one of these turbo-mixers is provided in each compartment ofthe tower, the turbine wheels being mounted on a common vertical shaftextending preferably axially of the tower, while the cooperating vanesare suitably known to chemical engineers.

in a counter current fixed in each compartment. For most eilicient 7.

gas absorption the course of the gas stream as it passes through theseveral compartments of the tower should be directed as nearly aspossible toward the center of each successive turbine wheel.

In some cases there may be a tendency toward foam formation in one ormore of the compartments, and I have found that this foam may besuccessfully broken down by any suitable agitating means. For example, asecond turbine wheel may be employed in each compartment if desired,which wheel may be similar to the other turbine wheel used in thecompartment, the second wheel being attached to the common axial shaftat a point preferably just above the surface of the liquid in thecompartment. Such turbine wheels function very satisfactorily as foameliminators, and do not need for this purpose the surrounding fixedvanes similar to those with which the other turbine wheels cooperate.

If it be desired to prepare the material on a batch basis instead ofcontinuously, I may useinstead of the tower, several tanks, preferablyvertical cylindrical tanks, in series, connected by gas conduits, inwhich tanks batches of the slaked lime suspension may be contained andinto which, successively, the carbon dioxide gas may be led. The gaspreferably passes through the several tanks in series, through the tankcontaining the most nearly carbonated material first, and through thetank containing the least carbonated last, the finished material beingthus produced successively in the several tanks according to the wellknown counter-current principle. The

.tanks should preferably be provided with agitators such asturbo-mixers, and the gas should be suitably, directed with respect tothese mixers as described under the tower description above. Alsosimilar foam eliminating means may be employed if necessary. If a seriesof tanks be used,

' the equipment required may sometimes be simplifled if the lime beslaked first, and then diluted if desired directly in the gas absorptiontanks, as this thus eliminates the necessity of separate lime slakingtanks, but it is usually not possible thus to slake lime to as heavy aslurry with agitators adapted primarily for gas absorption.

Thereaction may, if desired, especially if the conservation of gas isnot an important factor, be carried out as a batch process in anindividual container such as a tank, without the employment of thecounter-current absorption principle. In such event the gas is led intothe slurry preferably at or near the bottom of a tank which maypreferably be of'vertical cylindrical construction. Emcient agitatingmeans such as a turbo-mixer on a verticalshaft is provided to bringabout efiicient contact of gas with the liquid. A plurality ofturbo-mixer units may be provided in the tank, and the tank maypreferably be divided with reasonably close fitting substantiallyhorizontally disposed plates which may be cone shaped and perforated atthe center, and which provide a small annular space between the tankwall and the periphery-of each plate. These plates which are preferablyplaced between each two turboe confinedto one tank. with such anarrangement approximately 2 to 5 times the theoretical amount of carbondioxide required may be employed.

Whether tower, tanks, or a single tank be used,

ments-suitable for carrying out the reaction, and r of course it will beunderstood that not only may variations of the above be employed, butthat other known equipments suitable for gas absorption. such forexample as spray devices,.may be used for manufacturing my material.

Although I prefer; as stated above, to carry out the slaking of the limefirst, and then its carbonation, I may with a moderate degree of successstart the carbonation at the same time as the slaking, or before theslaking is complete, part of the carbonation thus taking place beforethe completionof the slaking. This procedure has the advantage of beingeconomical of time, but the quality of the product in certain cases doesnot seem to be quite so high as when the slaking is thoroughly completedbefore the carbonation has begun.

An operation or operations for the elimina tion or separation ofoversize particles should be performed either on the lime, preferablyafter finished material, because I .have found that in the process ofcarbonation, the percentage of oversize particles present in theoriginal raw material is reduced. Thus, a greater yield is obtained bymy preferred method, as undoubtedly some of what would otherwise beoversize parti cles are reduced to particles of usable size by theprocess of carbonation.

The material obtained from the ultra screening operation may be useddirectly, or it may be washed or otherwise treated, and dewatered and/ordried if desired by the use of known apparatus; If the material is tobedrled, the refln-. ing operation such as comminution, screening, airflotation, or the like may be practiced on the dried material ratherthan the screening and/or ultra screening of the wet material, but Iprefer as stated above, to perform the refining operation on the wetmaterial.

, n it be desired to further reduce the particle sizeor colloidize thematerial, such result can be accomplished by known means such as ballmills, rod mills, or other suitable mills.

For a source of carbon dioxide, I may use pure carbon dioxide such as isderived from lime kilns 'or the like, or I may even use a somewhat. lesspure carbon dioxide mixed with nitrogen such for instance as may bederived from the burning of coke, but I have found that it is entirelysatisfactory to employ ordinary boiler flue gases after suitablecleansing in a known manner. For example, flue gas may be drawn from aboiler flue,

duits andgas passed through a tower containing coke, preferablymoistened with water, primarily to remove the mechanical impurities; orif it be desired to remove certain of the impurities like sulphurdioxide, through a tower containing limestone preferably moistened withwater; and the purified gas may be used in the tower or tanks as indi-.cated.

The final carbonated material, for example in the case where dolomiticlime has been used, has a substantially definite composition and onewhich I have found is reproducible even with dolomitic lime derived fromtwo entirely different sources, such for instance as with dolomitic limederived from the Niagara formation of northern Ohio and with dolomiticlime derived from the western Massachusetts dolomitic lime formation.Such dolomitic limes after treatment to saturation with carbon dioxideas herein described yield products which have a residue on ignitionwhich equals approximately 50 to 51 per cent. of the original dry weightof the material.

of course as will be understood, if I carry the reaction far enough tocompletely carbonate the calcium but not to completion as far as theconversion of all the magnesium content of the lime into magnesium basiccarbonate is concerned, I obtain as the reaction product a mixture ofcalcium carbonate magnesiumhydroxide, and calcium carbonate magnesiumbasic carbonate.

This material will have the same general appearance under the microscopeas the completely carbonated material, acts technically in papersimilarly to a mixture of calcium carbonate ma nesium hydroxide andcalcium' carbonate magnesium basic carbonate, and this method may incertain cases-be a convenient method of manufacturing such a mixture.For example, the speed of absorption of the carbon dioxide appears toslow down with increase of time in certain cases with very thickslurries and thus under certain conditions it may not be economical tocarry the reaction to completion, in which case there is produced thematerial which is the mixture indicated above. It is to be understoodhowever that in any such mixture there is present in part at least thefinal material which I would produce had the reaction gone tocompletion, and the manufacture or use of such partially completedreaction product therefore falls within the scope of my invention.However, because of the improved quality of the material produced whenthe reaction is carried to completion, the latter is my preferredmethod.

As stated above I prefer to carry out the re-' action in making myproduct preferably at substantially atmospheric pressure. It will beunderstood however that the carbon dioxide gas used will necessarilyhave to be under sufficient pressure to overcome frictional resistancein concleaning apparatus and also the static head of the'suspensionin-theseveral tower compartments, in the tank, or in the series oftanks, whichever be used. Thus, in case the apparatus used comprises atower or series of tanks,

the several lower compartments of the tower,

as well as all but the last of the series of tanks, may be undersuflicient pressure to force the gas through the entire system.Providing the temperature be kept sufliciently high, such increase inpressure is of minor significance as previously set forth above.

For the purpose of providing the gas with the necessary pressure, Iprefer to use a gas blower or ump located preferably in the conduitleading from the main flue but before the gas cleansing tower or towers.By means of this blower the gas is given sufiicient pressure in slightexcess of atmospheric to force it through the gas cleaning system andalso through the tower or tanks containing the solid suspension to betreated.

As will be understood,-it is possible to reverse this procedure and suckthe gas through the entire equipment instead of forcing it, in whichcase, of course, a suction pump or fan may be attached to the gas outletof the tower or tanks, but such a procedure has the disadvantage thatair is drawn through any leaks in the system,

, which dilutes the gas, and forthis reason I prefer the pressure systemindicated above.

It will be apparent that if the gas containing carbon dioxide used inthis process be derived from the combustion in the air of carbonaceousmatter, such for example as coal, coke, oil, combustible gas, or thelike, and the absorption be carried out efliciently so thatsubstantially all of the carbon dioxide content of the gas be ab.-sorbed, the residual gas issuing from the absorption apparatus, such asthe tower, tanks or the like, will be substantially pure nitrogen, ornitrogen containing but a minor percentage of oxygen.

It is thus apparent that'as a by-product or a co-product of my processthere may thus be produced a substantially pure nitrogen, or a nitrogenY mixed with but a minor quantity of oxygen, and such nitrogen isparticularly suitable forum in the synthesis of 'ni'trogeneouscompounds, such for example as in the synthesis of ammonia. Thisnitrogen may be used directly from my process, or further purified ifdesired, or it may be used under pressure or liquefied if desired, andit thus becomes a very cheap source of nitrogen for further use. It isthus apparent that the process of manufacturingmy material may bedescribed as'a combined process for the manufacture of a pigment andnitrogen.-

As I have described my process above, I preferably start with a limecontaining magnesia.

' Instead of using lime as the starting material I may use, in a similaraqueous suspension, the pigment calcium carbonate magnesium hydroxide,such for example as is produced in conjunction withsodium hydroxide, i.e. an alkali metal hydroxide, from lime containing magnesia and sodiumcarbonate, i. e. an alkali metal carbonate, by the process described inmy prior Patent No. 1,415,391, of May 9, 1922. This material when madefrom dolomitic lime has a residue on ignition of approximately 61 to 62per cent. It is converted by my present process into a calcium carbonatemagnesium basic carbonate'having substantially the same crystallinestructure and substantially the same residue on ignition, namelyapproximately 50 to 51 per cent. as has the material produced whendolomitic lime itself is used as the starting material.

When used with calcium carbonate magnesium hydroxide as the startingmaterial, my process that the cost per ton of the carbonated material soproduced is usually less than the cost per ton of the original calciumcarbonate magnesium hydroxideproduced. If instead of using classified,screened or ultra screened calcium carbonate magnesium hydroxide as thestarting material,

the classifying screening and/or ultra screening operation which wouldordinarily be conducted magnesium hydroxide be deferred until subsequentto the carbonation process, the yield may be still furtherproportionately increased as explained above by the fact that during thecarbonation process some of the particles which would otherwise beremoved as oversize are reduced to usable size.

The above adaptation of my process, (1. e. the use of calcium carbonatemagnesium hydroxide as starting material) is particularly advantageousin cases where it is desired to produce a greater amount of pigment inproportion to sodium hydroxide, than is produced in the case wherecalbonate magnesium hydroxide (United States Patent No. 1,415,391),lime, containing magnesia, in excess of that amount which wouldordinarily be employed to produce a given amount of sodium hydroxide, i.e. lime in excess of that which under the conditions of the reactionwill have its calcium content substantially completely converted intocalcium carbonate. That process will then be conducted in such a mannerthat the final reaction product will not be substantially all calciumcarbonate magnesium hydroxide but rather calcium carbonate magnesiumhydroxide containing calcium hydroxide magnesi-' um hydroxide, i. e.part 'of the briginal slaked lime used in the process which may bedescribed as an alkaline compound of an alkaline earth metal. Thisreaction product is then subjected to carbonation by carbon dioxide,preferably by the above described process. The free lime present'reactsexactly as when lime alone is used as the starting material for myprocess as described originally herein, the calcium content being firstcarbonated to calcium carbonate, the lime thus being converted intocalcium carbonate magnesium" hydroxide. If it be desired merely toproduce as the final reaction product calcium carbonate magnesiumhydroxide, the process may be stopped at this point, there having beenproduced the calcium carbonate magnesium hydroxideformed in the originalreaction in addition to that produced by the carbonation of the calciumcontent of the excess lime containing -m agnesia which passed throughthe causticizing' process substantially unchanged. If it be desired tostop at this point, this point can readily be determined chemically bythe disappearance from the solid reaction product at such point'of wa-'ter extractible calcium hydroxide. Howeverif the product desired becalcium carbonate magnesium basic carbonate, the carbonation iscontinued and the calcium carbonate magnesium hydroxide reacts asdescribed directly above wherein the carbonation by the present processof calcium carbonate magnesium hydroxide (produced according to UnitedStates Patent No.

' 1,415,391) is described. The resulting product is calcium carbonate.magnesium basic carbonate of the desired-composition and microscopicpar- 4 ticle shape indicated above.

2,968,039 during the production of the calcium carbonate.

The carrying out of the reaction disclosed in my prior Patent No.1,415,391 with the use of an excess quantity of lime with subsequentcarbonation of the solid reaction product, thus allows the production ofa considerably increased proportion of pigment in relation to sodiumhydroxide produced in said process, such an increased proportion beingonly limited by the available size of tanks and other equipment.Furthermore, an added advantage of this procedure is that the presenceof free lime (usually in substantial amountythroughout the reactionresults in a higher percentage conversion of sodium carbonate intosodium hydroxide than would be the case were all the calcium hydroxidepresent in the lime converted directly to calcium carbonate in thereaction. Of course the excess free lime may be present in any amount,but

.usually it is not worth while commercially to use less than 10 percentvexcess.

The commercial manufacture of calcium carbonate magnesium hydroxide bythe reaction of a lime containing magnesia, for example dolomitic lime,and sodium carbonate is conveniently carried out in two stages in whatis substantially a counter current manner. In the first stage an excessof lime is present, whereas in the second stage an excess of sodiumcarbonate is present. While the changed procedure proposed herein usingexcess lime and subsequent carbonation with carbon dioxide is preferablycarried out by using originally an excess of lime larger than normal inthe first stage of'the process, it may also be carried out if desired byadding an excess of lime in the second stage of the process, or byadding lime to the finished material (calcium carbonate magnesiumhydroxide) and subjecting the mixture to the carbonation process. In anycase the lime should preferably be in well slaked condition when used.

A further modification of the process of manufacturingcalcium carbonatemagnesium hydroxide from lime containing magnesia and sodium carbonate,and one which results in considerable simplification, may be'practicedif excess lime be used in the process and subsequent carbonation withcarbon dioxide be employed. The process according to this modificationmay be practiced as a one stage process, that is, the reaction betweenthe sodium carbonate and the lime may take place only in one stage,corresponding roughly to the first stage of the process as now carriedout in the two stage manner except that salts, these solids, i. e.calcium carbonate mag-' nesium; hydroxide containing free lime, may bepassed directly to the carbonation process for conversion to calciumcarbonate magnesium hydroxide or calcium'carbonate magnesium basiccarbonate as desired. The washings may suitably be combined with theoriginally produced sodium hydroxide solution. As will be apparent thisis a great simplification of the operation as previously carried out,requiring as it does for causlcizing only one half of the apparatusformerly equired to conduct the operation in two stages, .nd thus willlead to a greatly decreased cost f operation. It is made possible ofcourse by the arbonation process which carbonates the cal- ;ium contentof the excess free lime which would lave been formerly accomplished bytreatment with sodium carbonate in the second stage of he process. Ifdesired the calcium carbonate nagnesium hydroxidegcontaining free limeneed iot necessarily be washed substantially free from :oda compoundsprior to carbonation (although ;hat is the preferred procedure), as thewashing :or the recovery of soda salts may take place on ;he finishedcarbonated material.

For certain uses it has been found desirable ;0 have associated with thecalcium carbonate magnesium hydroxide or the calcium carbonate magnesiumbasic carbonate, whichever be pro- :luced, a small amount, for example,one per cent. more or less, of a soluble alkaline salt such as sodiumcarbonate. Thus, it is at times desirable to so conduct the process asto permit this amount of soda to be present in the carbonated product.This may be accomplished, as will be apparent, either by regulating thewashing of the final material or by omitting this washing, or by actualaddition of soda salt to the material. It will also be understood thatif the carbonation product be made directly from lime containingmagnesia as the raw material, the same result may be accomplished bydirect addition of soda salt in the process or to the product.

It will be apparent that if, in any of the adaptations of thecarbonation procedure herein described, impure carbon dioxide gas beemployed, such for example as carbon dioxide containing sulphur dioxide,this will result in introducing a certain amount of impurity into thefinal reaction product. Ordinarily if the amount of impurity be small,it has little effect from a practical standpoint on the quality of thematerial produced; but it may be readily avoided if desired, forexample, by suitably purifying the gas used, as set out previouslyabove.

Having produced my desired calcium carbonate magnesium basic carbonateby any of the several modifications of my general method outlined above,I may now use the material in the manufacture of paper, either as afiller in the manufacture of filled paper, or as a coating pigment inthe manufacture of coated paper.

-In the manufacture of one modification of my .paper, namely filledpaper, I may mix the fibrous constituent or constituents with mymaterial suitably in a beating engine, subject the same to the usualmechanical treatment therein and if desired to subsequent treatment in arefining engine such for example as a Jordan, then after suitabledilution pass the mix on to a web-forming device, dry the web, andfinish it in the customary manner.

I may use any amo t of my material in proportion to the fibre asdesired, but inasmuch as my material is particularly well fitted for usein highly filled sheets, I prefer to use an amount which will give afiller content in the resulting sheet of approximately -30 per cent. Ifsuit-. able white water recovery systems are employed, the actualproportion of filler to fibre furnished in the beater need not bemuchgreater than that in the final paper, as substantially "all thefiller is recovered under such conditions, but if such systems are notemployed, it is of course necessary to use sumcient extra filler to makeup for the losses in the white waters.

In the manufacture of either substantially unsized or sized papercontaining my filler, there may be employed if desired any or all of theprocesses disclosed in my numerous issued patents or copendingapplications on the use of alkaline fillers in papermaking, particularlythose having to do with the addition of materials to the paper mix underconditions favoring the minimizing of the time and/or intimacy ofcontact of the constituents of the mix, or with the application ofmaterial to the paper web itself, and cross reference is hereby made tosaid patents and applications, a number of which are specificallymentioned below.

If it be desired to make unsized paper with my filler, no other materialbut the fibrous material and the filler need be employed, but usually itis highly desirable also to employ acidic material such as alumpreferably at the dilute stage as disclosed in my United States PatentNo. 1,803,646, issued May 5, 1931. Moreover it is at times verydesirable to employ starch, cross reference being made in this.connection to my United States Patent No. 1,831,928, issued November 17,1931. Furthermore the filler need not be added in the beater, but itmayand this is the procedure which is usually preferable-be added lateron in the process, e. g. at the dilute stage of the papermakingoperation, in accordance with the procedure disclosed in, my UnitedStates Patent No. 1,808,070, issued June 2, 1931.

If sized paper is to be made, suitable sizing agents such as rosin,paraflin or the like, suitably applied, may be employed for impartingwater resistance to the sheet. Such suitable'sizes and methods aredisclosed in numerous of my issued patents and co-pending applications,for example, Patents Nos. 1,803,643, 1,803,645, 1,803,647, 1,803,648,1,803,650, 1,803,651 and 1,803,652, all issued May 5, 1931; andapplications Serial No.

501,674, filed December 11, 1930, and Serial No.

534,461, filed May 1, 1931. d

The filled paper produced with my filler is distinctly superior inquality to that produced with calcium carbonate magnesium basiccarbonate produced by the carbonation of lime containing and apply thesame to the surface or surfaces of apaper body stock by the use of anysuitable paper coating machine. adhesives, such as starch, casein,- orthe like may be used. As will be understood, other organic materialsand/or inorganic materials, such as may be used in the manufacture ofcoated paper may be additionally incorporated in the mix if desiredprior to the application of the mix to the r body stock. The coatedpaper may then be dried and finished as usual. 3

Ihave foundthat my material, when produced from dolomitic lime, yields afilter pr''sscake of approximately 3'7 to 40 per cent. solid content,

Any of the customary according to the vacuum or pressure employed on thefilter press used. Furthermore I have found that the material appears toreduire a definitely larger percentage of adhesive for the production ofa coating of satisfactory strength than does my former calcium carbonatemagne sium basic carbonate, requiring in certain cases, with casein forexample, in the neighborhood of 35 per cent. air dry casein based on theweight of dry pigment. It will be apparent that when using a pigmentwhich is not dewatered to any greater extent than the above for coatingthe ordinary grades of book and magazine coated papers which require afairly heavy coating, due

care should be taken, if the adhesive be dissolved prior to admixturewith the pigment, that the adhesive be made up as afairly concentratedsolution, so as to produce a final mix of suitable concentration. Forexample, 1 part of casein in 4 parts by weight total casein solutionwill be a satisfactory concentration of adhesive to employ.

I have found when my pigment is the only pigment employed in a. coatingmix with an adhesive such as casein, that the mix has a tendency incertain cases to be somewhat difficult to spread and to brush mark whenit is used on a brush coating machine.

I have found, however, that by the use of the process disclosed in mycopending application Serial No. 428,231, filed February 13, 1930namely,the addition of a smallamount of acidic material such as alum to thematerial prior to blending it with the casein solution, that I have beenable to reduce the viscosity of the resulting mix and substantiallyovercome the tendency to spread poorly and brush mark. Approximately oneper cent. of alum based on the dry weight of the pigment is usually asatisfactory amount to employ.

When my pigment is used alone as the only pigment in a coating, itimparts an excellent degree of finish to the sheet. In certain casesthis does not seem to be so high a finish as is imparted by satin white,but it is substantially higher than the finish imparted by the calciumcarbonate magnesium basic carbonate made by carbonating dolomitic limeaccording to the process outlined in my above mentioned Patent No.1,595,416, and also higher than the'finish imparted by the use of thepigment calcium carbonate magnesium hydroxide disclosed in my Patent No.1,415,391. I have found that my pigment may be used as the only pigmentin a coating mix, or that it may be employed in connection with otherpigments ordinarily used in coating, such for example as clay, satinwhite, blanc fixe, calcium carbonate magnesium'hydroxide' or the like.It may be employed with any of the customary adhesives such as casein,modified starches and the like. Also the usual other ingredients of amix such as coloring matter, antifroth oils and the, like may beemployed. v

I have found that when my pigment is used ,in a coating mix with otherpigment such as clay, any tendency toward brush marking is greatlyminimized if not substantially eliminated. I have also found thatincertain instances'undel' 'such conditions there seems to be areduction in the adhesive requirements of my pigment, so that it thusbecomes desirable in certain cases to employ my material in conjunctionwith other pigment in the coating mix, apart from any consideration ofthe additive qualities which the several pigments mayimpart to such mix.v

I have found that the paper coated with m material has not only, asstated above, a substantially higher finish than paper coated with thecalcium carbonate magnesium basic carbonate produced as described in myPatent No. 1,595,416, but that the coated paper appears whiter in colorthan the paper coated with my former material, finishes more easily, hasbetter printing qualities, and is more opaque. These improvedcharacteristics, as will be well understood, are very desirable in acoated paper, and represent a distinct improvement in quality over thepaper for merly produced.

It is of course possible to employ paper filled with my material as thebody stock for paper coated with my material; but it is to be understoodthat paper filled with my material may, if desired, be coated with othermaterial; whereas any suitable paper other than that filled with mymaterial may, if desired, be employed as body stock to which to apply mycoating material.

When I speak of carrying out a.reaction within a giventemperature-range, I mean to include within that range the temperaturesmentioned as the limits ofthe range.

When I use the word paper herein, I use it in the broad sense to includeproducts of manufacture of all types and of all weights and thicknesses,which contain as an essential constituent a considerable amount ofprepared fibre and which are capable of being produced on a Fourdrinier,cylinder, or other forming, felting, shaping or molding machine.

The words coated paper in this specification are to be understood asreferring to the coated product produced by coating any type of paper.coming under the definition of paper given above.

Where I 'use the term alkaline earth metal, I intend it to includemagnesium.

. While I have described in detail the preferred embodiments of myinvention, it is to be under-' stood that the details of procedure, theproportions of ingredients, and the arrangement of steps may be widelyvaried without departing ,from the spirit of my invention or the scopeof the subjoined claims.

I claim:

1. In the manufacture of the pigment calcium carbonate magnesium basiccarbonate by the reaction of carbon dioxide on calcium carbonatemagnesium hydroxide, the improvement which consists in reacting calciumcarbonate magnesium hydroxide in the presence of water with carbondioxide and maintaining substantially all the magnesium content thereofin substantially water insoluble form during the addition of the carbondioxide by holding the temperature of the reaction mix during saidaddition approximately between C. and that of -boiling..

2. In the manufacture of the pigment calcium carbonate magnesium basiccarbonate by the reaction of: carbon dioxide on calcium carbonatemagnesium hydroxide, the improvement which consists 'in reacting calciumcarbonate magnesium hydroxide, in which the molecular ratio of themagnesium to the calcium is not in excess of substantially one to one,in the presence of water with carbon dioxide until substantially nofur-' ther carbon dioxide is absorbed and maintaining substantially allthe magnesium content thereof in substantially water-insoluble formduring the addition ofthe carbon dioxide by holding the temperatur ofthe reaction mix during said addition approximately between 60 C. andthat of boiling, whereby calcium carbonate magnesium basic carbonate isformed, substantially all the particles of which are of very finepoint-like shape' andwherein the molecular ratio oithe calcium to themagnesium radicle is less than three to one. 3. In the manufacture ofthe pigment calcium carbonate magnesium basic carbonate conjointly withalkali metal hydroxide, the improvement which comprises the steps ofreacting lime contai-ning magnesia with alkali metal carbonate in thepresence of water, substantially separating the solution of alkali metalhydroxide from the solid asso'ciatedtherewith, and reacting said solidin the presence of water with barbon dioxide and maintainingsubstantially all the magnesium content thereof in substantially waterinsoluble form during the addition of the carbon dioxide by holding thetemperature of the reaction mix during said addition approximatelybetween 60 C. and that of boiling. 4. In the manufacture of the pigmentcalcium carbonate magnesium basic carbonate conjointly with alkali metalhydroxide, the improvement which comprises the steps of reacting limecon-.

taining magnesia, in which the molecular ratio of the magnesium to-thecalcium is not in excess of substantially one to one, with alkali metalcarbonate in the presence of water, substantially separating thesolution of alkali metal hydroxide from the solid associated therewith,and reacting said solid in the presence of water with carbon dioxideuntil substantially no further carbon dioxide is absorbed andmaintaining substantially all the magnesium content thereof insubstantially water insoluble form during the addition of the carbondioxide by holding the temperature of the reaction mix during saidaddition approximately between 60 C. and that of boiling, wherebycalcium carbonate magnesium basic carbonate is formed, substantially allthe particles of which which comprises the steps of reacting slakedmetal carbonate in stantially separating-the solution of alkali metaltemperature of the reaction mix during said addition approximatelybetween 60 C. and that of are of very fine point-like shape and whereinthe molecular ratio of the calcium to the magnesium radicle' is lessthan three to one. b I

5. In the manufacture of the pigment calcium carbonate magnesium basiccarbonate conjointly with alkali metal hydroxide, the improvement whichcomprises the steps of reacting slaked lime containing magnesiawithaikali metal cartion mix during said addition approximately between0 c. and that of boiling.

6. In the manufacture of the pigment calcium carbonate magnesiumbasiccarbonate conjointly' with alkali metal hydroxide, the improvementlime containing magnesia, in which the molecular ratiov of the magnesiumto the calcium is not in excess of substantially one toone, with alkalie presence of water, subhydroxide from the'solid associated therewith,and reacting said solid in the presence of water with carbon dioxideuntil substantially no further carbon dioxide is absorbed andmaintaining substantially all the magnesium content thereof insubstantially water insoluble form during the addition of the carbondioxide by holding the boiling, whereby calcium carbonate -magnesiumbasic carbonateis formed, substantially all the' particles of which areof very fine point-like shape and wherein the molecular ratio ofthecalcium to the magnesium radicle is less than three to one.

7. In the manufacture of the pigment calcium carbonate magnesium basiccarbonate by thereaction of carbon dioxide on calcium carbonatemagnesium hydroxide, the improvement which consists in reacting calciumcarbonate magnesium hydroxide in the presence of water with carbon areaction of carbon dioxide on calcium carbonate magnesium hydroxide, theimprovement which consists in reacting calcium carbonate magnesiumhydroxide, in-which the molecular ratio of the magnesium to the calciumis not in excess of substantially one to one, in the presence of waterwith carbon dioxide until substantiallyflno fur-- 'ther carbon dioxideis absorbed and maintaining substantially all the magnesium contentthereof in substantially water insoluble form during the addition of thecarbon dioxide by holding the temperature of the reaction mix 'duringsaid addition approximately between C. and that of boiling, wherebycalcium'carbonate magnesium basic carbonate is formed, substantially allthe particles of which are of very fine point-like shape and wherein themolecular ratio of the calcium to the magnesium radicle is less thanthree to one.

9. In the manufacture of the pigment calcium carbonate magnesium basiccarbonate conjointly with alkali metal hydroxide, the improvement whichcomprises the steps of reacting lime containing magnesia with alkalimetal carbonate in the presence of water, substantially separating .thesolution of alkali metal hydroxide from the solid associated therewith,and reacting said solid in thepresence of water with carbon dioxide andmaintaining substantially all the magnesium content thereof insubstantially water insoluble form during the addition -of the carbondioxide by holding the temperature of the reaction mix during .saidaddition approximately between 80 C. and that of boiling.

' 10. In the manufacture of the pigment cal cium carbonate magnesiumbasic carbonateconjointly with alkali metal hydrdxidauthe mprovementwhich comprises the steps :of'reacting lime containing magnesia, inwhich the molecular ratio of the magnesium .to the calcium is not inexcess of substantially one to one, with alkali metal carbonate in thepresence of water, substantially separating the solution of alkali metalhydroxide from the solid associated therewith, and

reacting sai'dsolid in the presence of .water ,with carbon dioxide untilsubstantially no further carbon dioxide; is absorbed, and maintainingsubstantially all the magnesium content thereof in substantially waterinsoluble form during the addition of the carbon dioxide by holding thetemperature of the reaction mix during said addition approximatelybetween 80'. C; and that of boiling,

whereby calcium carbonate magnesium basic carbonate; is formed,substantially all the. particles of which are of very finepoint-likeshape and wherein the molecular ratio of the calcium to themagnesiumradicle i less than three to one.

11. In the manufacture of the pigment calcium carbonate magnesium basiccarbonate conjointly with alkali metal hydroxide, the improvement whichcomprisesthe steps of'reacting slaked lime containing magnesia withalkali metal carbonate in the presence of water, substantiallyseparating the solution of alkali metal hydroxide from the solidassociated therewith, and reacting said solid in the presence oiwaterwith carbon dioxide and maintaining substantially all the magnesiumcontent thereof in substantially water insoluble form during theaddition or the carbon dioxide by holding the temperature of thereaction mix during said addition approximately between 80 .C. and thatof boiling.

12. In the manufacture or the pigment calcium carbonate magnesiurnbasiccarbonate conjointly with alkali metal hydroxlde, the improvement whichcomprises the steps of reacting slaked lime containing magnesia, inwhich the molecular ratio of the to thecalcium is not in excess ofsubstantiallyone to one, with alkali metal carbonate in the, presence orwater, sub- Istantially separating the solution of alkali metalhydroxide from the solid associated therewith, and reacting said solidin the presence of water with carbon dioxide until substantially nofurtemperature of the reaction mix during said ad-- dition-approximatelybetweerr 80 C. and that 01,

boi1ing,.whereby calcium carbonate magnesium basic carbonate isfformed,substantially all the particles of which are of very iine point-likeshape and wherein the molecular ratio of the calcium to the magnesium'radicleis less thanthree to'one.

ROBERT mu. :0

